Bevy 0.9 - Changing a 2d material

2d_material.rs

use bevy::{prelude::*, sprite::MaterialMesh2dBundle};

#[derive(Resource)]
struct AlphaMaterialHandle(Handle<ColorMaterial>);
impl FromWorld for AlphaMaterialHandle {
    fn from_world(world: &mut World) -> Self {
        let mut materials = world.resource_mut::<Assets<ColorMaterial>>();
        Self(materials.add(ColorMaterial::from(Color::PURPLE)))
    }
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<AlphaMaterialHandle>()
        .add_startup_system(setup)
        .add_system(change_alpha)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    alpha_material: Res<AlphaMaterialHandle>,
) {
    commands.spawn(Camera2dBundle::default());

    // Circle
    commands.spawn(MaterialMesh2dBundle {
        mesh: meshes.add(shape::Circle::new(50.).into()).into(),
        material: alpha_material.0.clone(),
        transform: Transform::from_translation(Vec3::new(-10., 0., 1.)),
        ..default()
    });

    // Hexagon
    commands.spawn(MaterialMesh2dBundle {
        mesh: meshes.add(shape::RegularPolygon::new(50., 6).into()).into(),
        material: alpha_material.0.clone(),
        transform: Transform::from_translation(Vec3::new(10., 0., 0.)),
        ..default()
    });
}

fn change_alpha(
    alpha_material: Res<AlphaMaterialHandle>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    time: Res<Time>,
) {
    let alpha = (1. + time.elapsed_seconds().sin()) / 2.;
    if let Some(mat) = materials.get_mut(&alpha_material.0) {
        mat.color.set_a(alpha);
    }
}